Over the past 20 years, much effort has gone into developing evidence-based guidelines for clinical preventive services. The Canadian Task Force on the Periodic Health Examination, established in 1976, uses explicit criteria to grade the level of evidence used in, and the corresponding strength of, recommendations for including specific interventions in periodic preventive health examinations. The U.S. Preventive Services Task Force, established in 1984, uses the same criteria (Guide to Preventive Services, Williams & Wilkins, 1989), and the two task forces have collaborated....